The field of the invention is multilayer optical interference filters and devices comprising the same, particularly as useful in optical communication networks.
The use of multilayer optical interference filters has become ubiquitous in optical communication systems using wavelength division multiplexing (WDM). Such filters are the currently preferred method of separating or combining optical signal channels assigned to different wavelengths, according to the ITU grid, but propagating in common waveguides due to the minimum of insertion losses. However, the need to increase the number of signal channels that can be utilized within an existing optical fiber structure requires a decrease in wavelength spacing between adjacent channels. At the same time it is desirable to decrease the physical dimensions of active and passive components, such as filters within the switching fabric, as well as to decrease the component cost by integration of filters with other components. Accordingly, the performance requirements of optical interference filters have become more demanding. While the optical performance requirements can be met by increasing the number of layers, the physical properties of such thicker filters present significant challenges to reductions of the physical dimensions and integration with other optical components.
Another requirement of such filters is that the optical performance, that is the transmission characteristics as a function of wavelength, should not change over a large range of ambient temperatures. The thermal stability is characterized by the shift of the position of the maximum transmission region, or central wavelength position, and is less than 2 picometer (pm)/xc2x0 C. over the temperature range from 0xc2x0 C. to 70xc2x0 C.; preferably with a center wavelength (CWL) shift of less than about 0.5 pm/xc2x0 C. over the above temperature range.
The deposition of optical thin film materials onto a substrate to form an optical interference filter inevitably results in a net residual stress state in the multilayer structure. The residual stress may be intrinsic, that is an inherent property of materials as deposited in the form of a thin film.
Another source of residual stress is referred to as extrinsic, and results from the difference in thermal expansion coefficient between the substrate and thin film materials. The inherent properties of useful optical materials as deposited in multilayer thin films inevitably results in a residual stress state. As additional layers are added to increase the optical performance of the films, the net residual stress effect is increased flexure of the film and substrate, as the bending moment is the product of the thickness and stress. The residual stress presents limitations on the substrate that can be used. The substrate must be sufficiently thick to avoid its flexural deformation by the residual stress in the thin film layers.
Simply increasing the substrate thickness to minimize the peripheral deformation is unacceptable for a number of reasons;
The use of thicker substrates hinders reliable and efficient fabrication into smaller devices as well as the ability to integrate the filters with other active or passive optical components. Depending on the substrate optical properties, it also adds insertion loss. Stress induced limitations include, but are not limited to: bending of the filter resulting in defocus effects, especially in the reflected, or express signal; stress induced birefringence in the coating producing polarization dispersion loss (PDL) and position sensitive CWL variability.
Residual stress can also be a factor in device failure, and may prove to be a limitation to further increases in the signal power density, such as may be achieved through future developments in laser transmitters.
The importance of substrate selection in the thermal stability of interference filters is known. The linear coefficient of expansion of the substrate material may be chosen to either shift or stabilize the center wavelength multilayer dielectric bandpass filter with respect to a changing ambient temperature, as demonstrated by H. Takahashi in Applied Optics 34(4) pp. 667-675, 1995 (misspelled Takashashi in the original publication).
Since circa 1995, numerous high thermal expansion glass and glass/ceramic material formulations either have been adapted from other applications or specifically developed for commercial use as optical quality substrate materials. Representative commercial products for different types of glasses are:
i) xe2x80x9cF7xe2x80x9d, and xe2x80x9cDWDM-12xe2x80x9d by Schott Glass Technologies.
ii) xe2x80x9cWMS-01xe2x80x9d, xe2x80x9cWMS-02xe2x80x9d by Ohara Glass Company.
iii) xe2x80x9cWMS-1xe2x80x9d, xe2x80x9cWMS-12xe2x80x9d and xe2x80x9cWMS-13xe2x80x9d from Ohara Glass Company, among others.
The first four of these grades, groups i) and ii), are true glasses and owe their high coefficient of linear expansion in large part to their alloying composition, which includes a large fraction of alkali oxides. These alkali constituents unfortunately also degrade the environmental stability of the glass. Such instability can have serious deleterious effects on final device environmental performance.
The grades in group (iii), are representative of glass/ceramic composite structure, xe2x80x9cWMS-11xe2x80x9d, xe2x80x9cWMS-12xe2x80x9d and xe2x80x9cWMS-13xe2x80x9d are three such glasses. By proper choice of materials and processing conditions, substrates can be fabricated with the requisite high expansion coefficient. Such materials do not employ alkali constituents to attain their high expansion properties to the degree, as do more conventional glasses, and thus may be more environmentally stable. These materials also have the advantage of being generally stiffer, thus allowing them to better support the high stresses imposed by the thin film structures. However, their mixed glass/ceramic nature reduces their overall transmission, as the interfaces between the phases scatter visual and infrared light. Simple coatings have been removed from their substrate to form pigments with unique optical properties. In U.S. Pat. No. 4,434,010 (Ash et al.) removed a metal/dielectric filter from its carrier substrate, formed it into small flakes and demonstrated its use as a variable color reflective pigment material. Many other such examples abound. However, Ash discloses that the optical properties of the coatings are degraded, suggesting the method is only applicable for low performance applications.
Likewise, in U.S. Pat. No. 4,826,553 (Armitage et al.) invented a method for removing a mirror coating from its substrate and re-applying it wholly intact onto a second substrate in order to alter the dielectric mirror""s figure (curvature). In a later publication, Schmidt, et al (Photonic Spectra, May 1995) stated that such a method is applicable to lower performance telecommunication filters.
Solberg at al. in U.S. Pat. Nos. 5,944,964 and 5,930,046 teach a method of reducing stress by inducing crystallization of a high refractive index material to balance the compressive stress of silica. Two common high refractive index oxide materials are titania (titanium oxide) and zirconia (zirconium oxide). A common technique to densify and stabilize titania and zirconia thin film layers involves a post-deposition annealing process. Because the thin film layers are constrained by the substrate, which does not shrink the volume change from densification produces a tensile stress within the film layers. Indeed, the integrated tensile stress may exceed the integrated compressive stress of silica resulting in multilayer thin film stacks having an excessive net tensile stress also resulting in loss of mechanical integrity or poor optical performance. In addition, the crystallization that occurs during annealing may contribute to increased optical scatter, which also degrades optical performance.
The process combination taught by Solberg minimizes many of the detrimental effects of crystallization to exploit it as a means for reducing net stress arising from the thin film deposition conditions. Unfortunately, the method is inapplicable to band pass filters used in WDM filters as the crystallization process invariably introduces scatter and stress birefringence, thus lowering transmission and increasing PDL.
Another requirement of WDM filters is that the optical performance, that is the transmission characteristics as a function of wavelength, should be spatially uniform over the region illuminated by the optical signal beam. Having discovered that higher performance WDM filters exhibit a strong spatial non-uniformity in transmission at the peripheral regions, this non-uniformity has heretofore proved a limitation in the miniaturization of filters for advanced packaging applications. Although it might be highly desirable in some instances to reduce the filter to not much larger that an optical beam size of less than about 500 microns, attempting to do so would otherwise degrades the device performance. Unless the spatial non-uniformity at the edges is somehow corrected, the optical signal channels in the beam will be moderated according to the average filter performance over the area the beam illuminates.
Accordingly, there is a need for optical multilayer thin film filters that have stable wavelength transmission characteristics over a broad range of ambient temperatures, having high transmission at the center wavelength, exhibiting low scatter to minimize insertion loss as well as low polarization dispersion loss (PDL)
There is a corresponding need for reducing the physical dimensions of such optical interference filters such that they can be integrated with other passive and active optical compounds such as lenses, switches, lasers, modulators, photodetectors and the like. Accordingly, another object of the invention was to discover and eliminate the source of this non-uniformity in WDM filters. Having determined that the spatial non-uniformity correlates with residual stress, yet another object of the invention has been to preserve the properties of such optical interference filters that make them desirable for WDM applications, while substantially reducing the residual stress.
Not wishing to be bound by theory, we have discovered that very thick multiple layer optical interference filters have a spatial non-uniformity in transmission that correlates with the high residual stress. This non-uniformity proved a limitation in the miniaturization of filters for advanced packaging applications, as it manifests itself at the peripheral regions of the filters. Thus attempting to miniaturize such filter to accommodate a small optical beam results in spatial variations within the beam area.
In one embodiment of the invention a freestanding optical filter is formed by first depositing a sequence of thin film layers on a first substrate. Subsequent removal of the thin film layers from this first substrate as a monolithic element relieves the residual stress therein This embodiment proved enabling for further miniaturization of the optical filter components.
The release of the optical coating from its substrate provided several benefits, as the substrate formerly had introduced losses and/or aberrations of the light beam. While such a free standing filter suffered from reduced thermal stability, another aspect of the invention is reattachment to a second substrate so that such smaller filters would exhibit both thermal and spatial uniformity of transmission and reflection over a larger region of the device. The second substrate and its method of attachment provide a means to thermally stabilize the filter.
In yet another aspect of the invention a subassembly in which the optical path through the multilayer dielectric interference filter is free of an intervening second substrate, yet provides a center shift of less than 2 pm/xc2x0 C. over the temperature range from 0xc2x0 C. to 70xc2x0 C. In this embodiment the second substrate is a frame or annulus having an opening that defines an optical aperture. Thus, a portion of the multilayer dielectric interference filter is unsupported over this optical aperture. The frame is selected to provide the thermal stabilization of the center wavelength position.